Since the 1970s there has been a rapid expansion of salmon farming with intercontinental movement of fish; stocks have been moved within their North Atlantic range and into the North Pacific and Southern Hemisphere temperate regions. This anthropogenic intervention has led to a situation conducive to the emergence of diseases. Movement of fish and equipment leads to the large-scale transmission of pathogens. Population intensification and stress associated with hatcheries and farms creates an environment in which these pathogens can become established and evolve. An example of such a pandemic is the recent emergence of Infectious Salmon Anemia, which since the 1980s has spread to Norway, Canada, Scotland, the Faroes, Chile and the USA. The factors behind the spread and virulence of this disease are reviewed, with particular reference to the UK and in the light of other emerging diseases. Means of restricting the emergence of diseases are discussed, in the existing and potential industry.